Acoustic echo generally refers to a phenomenon in which a sound wave originated from a sound source is reflected by a surface of an object and returned to the sound source. An example of acoustic echo, which can be often found in the everyday life, includes a natural echo with single reflection. A direct sound is the opposite of the acoustic echo. The term “direct sound” refers to a directly heard sound without being reflected by the surface of an object. In other words, the acoustic echo indicates a reflected sound that arrives about 0.05 or longer second behind the direct sound in terms of the hearing sense. Therefore, the echo sound and the direct sound are heard with a time difference. In the place with multiple reflecting surfaces, such as a room and a vehicle interior, the reflection is repeated several times in various directions, generating a complex echo sound. This is an example of a multiple reflection echo, also known as a reverberation.
With the development of communication technology, the communication system is evolving from a wired communication system into a wireless communication system. In order to provide a convenient call environment, there has been proposed a hands-free technique in which a user talks over the phone using a microphone and a speaker instead of the earpiece and mouthpiece. The hands-free technique is applicable to a car hands-free phone, a remote conference system, a speaker-phone system, mobile and stationary wireless telephony devices, and so on.
In the communication system where voice communication between the user and the communication device is performed through the speaker and the microphone, it is necessary to take into consideration the fact that a part of the voice or acoustic sound output from the speaker is input to the microphone. Therefore, the acoustic echo component should be taken into account to provide a smooth call. In a full-duplexing hands-free voice communication system, if the acoustic echo component is not appropriately canceled, a far-end user hears back his/her own voice after a lapse of a predetermined time, together with a voice of a near-end user. In other words, the far-end user is inconvenienced by an echo phenomenon during a call.
The acoustic echo occurs because a far-end user's signal output from the speaker is input to the microphone via an acoustic echo path, together with a noise, and then transmitted back to the far-end user. As a result, the far-end user receives the undesired echo signal along with the near-end user's signal.
The acoustic echo path of the echo signal undergoes a frequent change with the passage of time when a mobile terminal operates not only in a normal voice call mode but also in a video conference mode or a speaker phone mode. For example, the acoustic echo path undergoes a change even when a participant of the conference moves his/her head, arm and shoulder during the video conference.
Therefore, many conventional mobile terminals use an Automatic Gain loss Control (AGC) to cancel the echo phenomenon. In one implementation, the AGC then reduces echo based on the reduction of energy (or power) of the microphone signal depending on the energy of the loudspeaker signal or on an estimated energy of the echo. This technique can be seen as the application of a gain G on the energy (or power) of the microphone, G depending on the energy of the loudspeaker or of the estimated energy of the echo. The gain G is in practice applied on successive frames m in the time domain, in the frequency domain or in any time-frequency domain.
Although the foregoing technique may be effective in removing echo, the far-end user also hears noise muting as a result of the echo removal. This noise muting is also known as noise modulation. One technique to mitigate noise modulation is to apply a threshold to the gain G. This is effective in reducing the noise modulation, but the threshold application may reduce effectiveness of the echo cancelation. Another technique to mitigate noise modulation is to use noise injection. This technique adds artificial noise, or so called “comfort noise” with the same energy as the surrounding noise suppressed by the echo cancelation. The disadvantage of noise injection using white noise or pink noise relates to the frequency characteristic of the injected noise compared to the background noise eliminated by the echo cancelation. The mismatch between the frequency characteristics of the injected noise and the actual background noise leads to the irritating effect that the far-end speaker hears the noise changing when he/she speaks. That is, the user hears white or pink noise when he/she speaks and when silent, the user hears the actual background noise.